As worldwide petroleum deposits decrease, there is rising concern over shortages and the costs that are associated with the production of hydrocarbon products. As a result, alternatives to products that are currently processed from petroleum are being investigated. In this effort, biofuel such as biodiesel has been identified as a possible alternative to petroleum-based transportation fuels. In general, a biodiesel is a fuel comprised of mono-alkyl esters of long chain fatty acids derived from plant oils or animal fats. In industrial practice, biodiesel is created when plant oils or animal fats are reacted with an alcohol, such as methanol.
For plant-derived biofuel, solar energy is first transformed into chemical energy through photosynthesis. The chemical energy is then refined into a usable fuel. Currently, the process involved in creating biofuel from plant oils is expensive relative to the process of extracting and refining petroleum. It is possible, however, that the cost of processing a plant-derived biofuel could be reduced by maximizing the rate of growth of the plant source. Because algae is known to be one of the most efficient plants for converting solar energy into cell growth, it is of particular interest as a biofuel source. Importantly, the use of algae as a biofuel source presents no exceptional problems, i.e., biofuel can be processed from oil in algae as easily as from oils in land-based plants.
While algae efficiently transforms solar energy into chemical energy via a high rate of cell growth, a high rate of cell growth alone is not sufficient to make algae-based biofuel cost efficient. Rather, the efficient production of biofuel further requires a maximization of the rate of oil production within the algae cells. Currently, the production of biofuel from algae is limited by a failure to maximize both algae cell growth and the production of oil within the algae cells. Specifically, the conditions necessary to facilitate a fast growth rate for algae cells have been found to hinder the production of oil within those algae cells. Likewise, the conditions necessary to facilitate a fast rate of oil production within algae cells have been found to hinder the growth of the algae cells.
In greater detail, the biochemical process of photosynthesis provides algae with the ability to convert solar energy into chemical energy. During cell growth, this chemical energy is used to drive synthetic reactions, such as the formation of sugars or the fixation of nitrogen into amino acids for protein synthesis. Excess chemical energy is stored in the form of fats and oils as triglycerides. Therefore, it can be seen that cell growth and triglyceride production compete for the same chemical energy. As a result, the simultaneous rates of growth and oil production are inversely related. In the past, emphasis has been on algae cell growth and the resulting low oil production has been accepted as a necessary condition.
In light of the above, it is an object of the present invention to provide a system and method for maximizing both the cell growth of algae and the production of oil within the algae cells. For this purpose, a number of systems have been developed, such as those disclosed in co-pending U.S. patent application Ser. No. 11/549,541 for an invention entitled “Photosynthetic Carbon Dioxide Sequestration and Pollution Abatement,” co-pending U.S. patent application Ser. No. 11/549,552 for an invention entitled “High Photoefficiency Microalgae Bioreactors” and co-pending U.S. patent application Ser. No. 11/549,561 for an invention entitled “Photosynthetic Oil Production with High Carbon Dioxide Utilization,” which are filed concurrently herewith and assigned to the same assignee as the present invention, and are hereby incorporated by reference. Another object of the present invention is to provide a two stage system with a first stage for maximizing algae cell growth and a second stage for maximizing oil production within the algae cells. Still another object of the present invention is to provide a system for growing algae cells in which oil production in the cells is selectively triggered. Another object of the present invention is to provide a two stage reactor system for growing algae with a high oil content that defines a flow path for continuous movement of the algae during cell growth and oil production. Yet another object of the present invention is to provide a system and method for producing algae with high oil content that is simple to implement, easy to use, and comparatively cost effective.